So, pinning is an extremely complicated topic that I've always wanted to write a blog post about, but have never had the time to do it. It happens fairly regularly that we have to assist a company that has painted themselves into a corner with a poorly thought out pinning scheme.
In my experience, it's mostly bad implementations of pinning that cause most of the problems, not pinning itself. I've seen people pin leaf certificate serial numbers, for example. That's guaranteed to end badly. If you do feel the need to pin, please make sure you've thought through all the possible certificate replacement use cases and make sure you have designed, implemented, AND TESTED them to make sure they work. All too often, a customer complains things stop working when they replace their certificate ... but no one at the company is aware what exactly is being pinned or why. Use pinning with care. -Tim > -----Original Message----- > From: dev-security-policy <dev-security-policy-boun...@lists.mozilla.org> On > Behalf Of Ryan Sleevi via dev-security-policy > Sent: Wednesday, August 14, 2019 2:08 PM > To: Nuno Ponte <nuno.po...@gmail.com> > Cc: mozilla-dev-security-policy > <mozilla-dev-security-pol...@lists.mozilla.org> > Subject: Re: Use of Certificate/Public Key Pinning > > On Tue, Aug 13, 2019 at 11:12 AM Nuno Ponte via dev-security-policy < dev- > security-pol...@lists.mozilla.org> wrote: > > > Dear m.d.s.p., > > > > I would like to bring into discussion the use of certificate/public > > key pinning and the impacts on the 5-days period for certificate > > revocation according to BR §4.9.1.1. > > > > Recently, we (Multicert) had to rollout a general certificate > > replacement due to the serial number entropy issue. Some of the most > > troubled cases to replace the certificates were customers doing > > certificate pinning on mobile apps. Changing the certificate in these > > cases required configuration changes in the code base, rebuild app, QA > > testing, submission to App stores, call for expedited review of each > > App store, wait for review to be completed and only then the new app > > version is made available for installation by end users (which is turn > > are required to update the app the soonest). > > > > Meeting the 5-days deadline with this sort of process is > > “challenging”, at best. > > > > A first approach is to move from certificate pinning to public key > > pinning (PKP). This prevents the need to update the app in many of the > > certificate replacement operations, where the public key is reused and > > the certificate can be replaced transparently to the app (generically, an > “User Agent” > > doing PKP). > > > > However, in the event of a serious security incident that requires > > re-key (such as key compromise), the certificate must be revoked in > > less than 24 hours (for the benefit of everyone – subscriber, relying > > parties, issuing CA, etc). It’s virtually impossible to release a new > > app version within this timeframe. And this, I think, make a very > > strong point against the use of PKI. > > > > On the other side, PKP is a simple yet powerful and effective > > technique to protect against MITM and other attacks. It seems to be > > widely used in apps with advanced threat models (mobile banking, > > sensitive personal information, etc) and there are many frameworks > > available (including native support in Android via Network Security > Configuration [1]). > > > > There are several possible mitigation actions, such as pinning more > > than one public key to have more than one certificate to quickly > > rollover in case of a revocation. Even then, it is very likely that > > all the redundant key pairs were generated and maintained by the same > > systems and procedures, and thus all of them will become effectively > compromised. > > > > Ultimately, it may become common practice that 1) PKP frameworks are > > set to bypass revocation checks or 2) PKP is done with private > > certificates (homemade, self-signed, managed ad-hoc with no CRL/OCSP > > services). Does any of this leads to a safer Internet? > > > > I don’t expect this thread to end up into an absolute conclusion > > advocating for or against, but opening it to discussion and > > contributions may help to document possible strategies, mitigations, > > alternatives, pros & cons, and hopefully provide guidance for an educated > decision. > > > > Best regards, > > > > Nuno Ponte > > Multicert SA > > > > Nuno, > > Thanks for raising this. I appreciate that a CA is attempting to provide > guidance > to their Subscribers, since, as you note, the CA is still beholden to the > Baseline > Requirements, and the Subscriber still beholden to their Subscriber > Agreement. > > As one of the co-authors of the HTTP Public Key Pinning RFC, RFC 7469, I would > readily encourage your Subscribers not to pin at all. However, if they are > going > to pin to anything, then it: > 1) Should be to the public key, not certificate > 2) Should only be to the root > 3) Should require an agreement with the root that the Root (i.e. Multicert) > will > disclose all possible certificate paths and root certificates that are > unexpired > and may exist. > - This includes cross-signed certificates > > Note that mobile devices, the scenario you discuss for pinning, already > largely > do not implement revocation checks and do not offer a way for applications to > control it, so I don't think that should be a concern. > > Of course, many of these techniques predate improvements to the ecosystem, > such as CAA and Certificate Transparency. Many of the concerns that > motivated HPKP, which subsequently evolved into the aforementioned OS > APIs, were predicated on an assumption of a CA that has lax validation > methods. > Yet techniques such as CAA exist to give site operators meaningful control > - and to ensure that any CA that violates such checks, and ignores CAA, is > unambiguously at risk of being distrusted by the entire platform. > > If there's concern about the DNS registrar being hacked, the Subscriber has > the > option of choosing a better registrar before hand, without resorting to PKP, > and/or transitioning to registries with better security practices overall. In > general, this means avoiding ccTLDs, which are known lax due to their lack of > any formal IANA or ICANN agreements with respect to operations. Certificate > Transparency offers a compliment to this, ensuring that even if the Registrar > or > Registry is compromised, misissued certificates can be promptly detected for > subsequent investigation and remediation. > > However, as to your question about private PKIs potentially being more secure > - it is indeed possible this may be the right answer! Organizations which > desire > complete lifecycle control over their certificates may benefit from the use of > such private PKIs - while also being wholly responsible for the operational > security of such, which naturally will be something their regulatory oversight > bodies will take great interest in. > > Beyond that, what Matt and Tom said is absolutely correct. Making use of > multiple (distinct) TLS endpoints, even on the same IP, is absolutely crucial > for > reducing these risks. > > As a CA, the best thing you can do is to make sure your Subscribers know that > they are taking on operational risk when they pin, and the CA is not > empowered to assist in resolving that. > _______________________________________________ > dev-security-policy mailing list > dev-security-policy@lists.mozilla.org > https://lists.mozilla.org/listinfo/dev-security-policy
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